This invention relates generally to large turbomachines; and, in particular, this invention relates to turning gears for steam turbine-generator units.
A turning gear is a motor-driven gear train, the primary function of which is to slowly roll a turbomachine rotor prior to starting the unit with steam and after shutdown to ensure that the rotor remains straight. A turbine rotor will cool unevenly if permitted to remain stationary for a period of time, and this may cause bowing to occur which would result in severe vibration and rubbing of stationary parts when the turbine is restarted. A secondary function of the turning gear is jogging a rotor to a new position during assembly, balance operations, and inspections.
A turning gear is actuated as a rotor coasts to a stop after a period of operation. A short time prior to a zero speed indication from the turbine, the turning gear motor is energized. When the zero speed indication is received, a turning gear pressurized fluid cylinder rotates a clash pinion carrier (carriage) to a position adjacent a bull gear on the turbine rotor. The clash pinion itself, rotated by the turning gear motor, through a gear train, is forced into mesh with the rotor bull gear.
A turning gear apparatus of the type described above is shown in U.S. Pat. No. 3,919,894 issued Nov. 18, 1975 to Keeter, et al., and assigned to the present assignee. The Keeter, et al., patent addressed the torque requirements to engage the clash pinion with the rotor bull gear by employing a small pre-engagement motor in tandem with the main turning gear motor. While this invention has been effective in reducing peak dynamic contact stresses in turning gears for large turbomachines, the very large forces involved in overcoming static friction in a stationary turbomachine rotor may result in structural damage to the turning gear assembly and damage to the gears because of misalignment due to movement and distortion of the turning gear housing Massive and rigid structures have been required to resist the reaction forces on the turning gear, and excessive power has been required to overcome the static friction in the rotor If the turning gear is located such that the drive pinion force on the bull gear adds to the downward force on the rotor bearing, the magnitude of the breakaway torque is even greater.
Accordingly, one object of the present invention is to provide an improved turning gear engagement mechanism in which the static friction due to the weight of the turbomachine rotor is decreased.
Another object of the invention is to provide an improved turning gear engagement device, in which the total force on the pinion carriage is reduced and which improves the distribution of force on the pinion carriage, thus requiring a less massive structure.
Still another object of the invention is to provide an improved turning gear engagement device which reduces the necessary power to turn the same size rotor.